For years, scientists believed Al Pacino and Robert De Niro couldn't occupy the same space at the same time. No one could imagine that much coglioni onscreen. The pair came close in The Godfather Part II, a 40-year rift separating Michael Corleone and the younger version of his father. Michael Mann would disprove the theory once and for all with 1995's Heat. He cast Pacino as driven Lt. Vincent Hanna, and De Niro as Neil McCauley, a career criminal ripped from headlines. And he had them confront at one of Los Angeles' most famous restaurants, Kate Mantilini, manifesting a nucleus to power his epic crime picture. Heat is about the duality of human instincts. In that scene, between two legendary actors, the theme bursts off the screen. It is biblically awesome. And watching the film again as part of the Toronto International Film Festival's 20th anniversary tribute, Mann can't help but be enraptured by his own success.

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"It's a different experience every time you see it," Mann told the audience as the credits rolled. This time, the director found himself mesmerized by his ensemble's eyes, expanded by an 80-foot screen and projected on crisp 35mm. "[I could] just hold on their eyes. There's no performance. We were all so into character. You were the movie as well as the individual character."

Clocking in at 170 minutes, Heat chronicles the sprawling cat-and-mouse game between the LAPD and McCauley's crew, and the personal lives sacrificed in the process. Mann, who also wrote the script, diligently carves out his characters (the film exists in the gray between feature film and a season of binge-watched TV). Heat is known for its 10-minute shootout, a botched heist erupting on the streets of L.A., but it's cemented by interwoven intimacy. There's time for Hanna to stop at home, bicker with his third wife, watch his stepdaughter drift into depression, and sense that it's all provoked by his never-ending pursuit of justice. There's time for McCauley to sit his number-two, Chris Shiherlis (Val Kilmer), down for a life talk, question Chris' adulterous wife, and walk away realizing there's no one in his life to even screw over. He's forever alone. Mann sympathizes with all of his characters, even when they shoot innocents at point-blank range. Morality and criminality are fluid. Heat is the only opera to swap out melodious sopranos for a booming M733.

Mann spent years preparing for Heat. Every Friday and Saturday for nine months, he rode along with LAPD plainclothes commander Tom Elfmont, answering calls from 9 p.m. to two in the morning. He visited Folsom State Prison, speaking to inmates who helped shape McCauley's mentality. He helped Ashely Judd meet women in real estate who, 50 years earlier, had been "turning tricks on Van Nuys Boulevard" to support their kids while their husbands were in prison. The characters in Heat were not characters. They were people extracted from the real world, injected into guys like Pacino and De Niro, then set back loose to play out however it made sense.

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"Life is filled with what the outside sees as contradiction, but it's not really contradictory," Mann says about his dimensional heroes and henchmen. His reference point was Ford Madox Ford's 1915 novel Good Soldier, which wrestled with character subjectivity. When the audience followed McCauley, they'd enter his bubble, understand his dubious motivation. "Neil commits to Eady [Amy Brenneman]. He's abandoned the kind of non-attachment, rigid, functional-versus-reward equation, and he's initially successful in spontaneously convincing her to go with him. At the same time, he's made himself vulnerable to be blown aside off his course by emotional vengeance. Which 30 minutes earlier in this film, or never in a million years, would he have gone for."

Many filmmakers have aspired to Mann's brand of action cinema, from Christopher Nolan and Nicolas Windig Refn to Nic Pizzolatto, who reached for Heat with True Detective's dense second season. It exists in the fire between Pacino and De Niro, in the surprising heaviness of supporting players—the recent re-watch reveals Dennis Haysbert's Donald as the film's most tragic player—and in the surgical precision of its action. McCauley and Hanna are experts at what they do. Mann's trenchant style and research addiction empower them. According to Mann, before filming the bank heist, he, De Niro, Kilmer, and Tom Sizemore cased a working bank. Armed with unloaded weapons, and with only the security guards in the know, the squad covertly plotted a heist that would only exist in fiction.

"Choreography has to tell a story or it's just gratuitous actions," Mann says. Which is why he ingrained every beat of the shootout sequence into his cops and robbers. The Heat team recreated the L.A. battleground at the county sheriff's ranch. The actors trained with live ammo to give them a sense memory when using blanks on set. It had to go this deep. The crew could only film the shootout on the weekends due to L.A. traffic. On business days, they moved on to other scenes, a jarring experience for a film bent on concentrated performances.

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Today, computer graphics and backlot sets and stand-in cities would take the place of Mann's L.A. At his Q&A, the director couldn't help but feel nostalgic for what he gave over to Heat. "There is something to be said for doing it for real," he says. There was the obvious wow factor—yes, those planes in the final chase are really flying over De Niro and Pacino—but also something more granular. "I start to look for what's anomalous in a character's behavior, because that's going to increase the specificity of identity." Jon Voight's bolo tie, the way De Niro holds his Campari as he ends Trejo's life, every line about women's asses that explodes out of Pacino's mouth—Mann fed on all of it. "Audiences are way smarter than we know we are. Perceptually we're brilliant. Our brains register truth or naturalness way before our cerebral cortex, before you can name it, within a couple hundred-thousandths of a second, but perception is quite brilliant and so I've always felt it was my responsibility, my job to use all these tools and constantly try to use these tools to intensify the experience."

After two and a half hours of power dynamics, it's hard to watch Heat end. One of them has to die. And you feel torn. Mann says he knew how it had to end, even if both Hanna and McCauley fascinated him. "What I came to later was to the perfect equivalence of the connection. It occurred to me that Neil was fortunate enough to die in contact with the only other guy on the planet who was really similar, almost like him, and understood him totally. That became the end of a dialectic which I then reverse-engineered into a lot of previous scenes which preceded it. I could build off that final moment."